1. Field of the Invention
The present invention relates to thermal fixation in printers and similar image formation apparatuses and particularly to detecting the temperature of a hot roller.
2. Description of the Background Art
Laser printers corresponding to conventional image formation apparatuses receive image data from a host and process the data at an image formation unit. More specifically, a photosensitive material is exposed to laser light to form an electrostatic latent image which attracts charged toner and is developed, and a sheet is brought into close contact with the photoreceptor to receive the toned image which is in turn fused to provide a permanent image.
In connection with the thermal fixation a conventional laser printer provides thermal fixation at a fuser internally provided with a movable fusing member implemented by a hot roller having a surface with a thermistor attached thereto. The thermistor is a temperature detection device decreasing in resistance as it is heated, and is generally used to detect the hot roller's surface temperature. It is used to exert control to set the hot roller's surface temperature for example around 180 to 200° C., which allows efficient thermal fixation.
In connection with this temperature control, the thermistor or the like provides analog output, which may have its voltage value linearly read for example via a microcomputer or ASIC's analog port and from the read voltage value the temperature may be detected and the surface temperature may thus be controlled. However, the analog port, having a function reading the voltage value linearly, is controlled in a complicated manner and is also an expensive component. Accordingly, it is suggested to replace the analog port with an AD converter converting an analog output to a digital output and detect temperature from the output of the AD converter.
For example, Japanese Patent Laying-Open No. 07-055589 discloses an AD converter for temperature control wherein a temperature detected of a temperature measuring thermistor is detected by the AD converter. More specifically the document discloses that a plurality of resistors are used so that a threshold value obtained by resistance division can be used to detect temperature at a plurality of temperature measuring points.
The AD converter for temperature control that employs a plurality of resistors to measure temperature at a plurality of temperature measuring points, however, has variation between the resistors, and temperature cannot be detected with precision.
Furthermore, if the number of temperature measuring points is increased a plurality of resistors different in resistance must accordingly be provided. This disadvantageously results in increased number of components and hence increased area for mounting.